1. Field of the Invention
The present invention relates to a high-density optical disc, and more particularly, to a high-density compact optical disc capable of securing good recording/reproducing characteristics and high-density recording capacity.
2. Description of the Related Art
Optical discs are generally adopted as information recording media of optical pickups which are capable of recording/reproducing information in a non-contact manner. The optical discs are divided into compact discs (CDs) and digital versatile discs (DVDs) according to information recording capacity. Here, mini discs (MDs) having a diameter of 65 mm or less may further be included.
Such recording discs may be subdivided into read only discs and rewritable discs. The read only discs include CD-ROM (Read Only Memory), DVD-ROM, and the like. The rewritable discs include CD-RW, CD-RAM (Random Access Memory), DVD-RW, DVD-RAM, and the like.
A CD has the whole thickness of 1.2 mm and is made from polycarbonate (PC). The CD is reproduced by a laser diode having a wavelength of 780 nm, a track pitch thereof is 1.6 μm, and has a single surface with an external diameter of 120 mm and a recording capacity of 650 MB.
A DVD can be formed to have a thickness of 1.2 mm by adhering a polycarbonate stiffening plate of a 0.6 mm thickness onto an information substrate of a 0.6 mm thickness so as to be compatible with CD drives. If a DVD is a single-sided disc having a diameter of 120 mm, recording capacity thereof is 4.7 GB. If a DVD is a double-sided disc, two discs having a thickness of 0.6 mm may be put into contact to have recording capacity of 9.4 GB. A DVD is reproduced by a laser diode having a wavelength of 635/650 nm and an object lens having a numerical aperture of 0.65, and track pitch thereof is about 0.74 μm. A DVD of a 80 mm diameter has recording capacity of 1.47 GB corresponding to recording capacity of two CDs.
A high-density (HD)-DVD is single-sided, has a diameter of 120 mm, and a recording capacity of about 250 GB. The HD-DVD is recorded/reproduced by a laser diode having a wavelength of 400 nm and an object lens having a numerical aperture (NA) of 0.85, and track pitch thereof is about 0.3 μm. An HD-DVD having a diameter of 80 mm has a recording capacity of about 7.8 GB.
An HD TV requires a disc capable of recording/reproducing an HD movie of two hours and fifteen minutes, has a diameter of about 120 mm, and has a recording capacity that is not standardized, but where about 23 GB of data can be recorded/reproduced.
The standards of a CD and a DVD are listed, respectively, in table 1.
TABLE 1CDDVDDiameter (mm)120120Thickness of whole disc (mm)1.21.2Thickness of information substrate (mm)1.20.6Track pitch (μm)1.60.74Minimum pit size (μm)0.830.4Wavelength of laser beam (nm)780635/650Numerical aperture (NA) of object lens0.50.65Capacity of single substrate (GB)0.654.7(1.47)
A disc 1, such as a CD or a DVD, shown in FIG. 1 is divided into a central hole 10, a clamping area 20 which stably places the disc 1 on a turntable of a reproducing drive that will be described later, a lead-in area 30 on which information related to the disc 1 is recorded, a data area 40 on which user data is recorded, and a lead-out area 50 which is an end portion of an area for recording data.
The lead-in area 30 can record reproduction only data, such as the size of the disc 1, the number of tracks of a reading layer, copy protection information, or the like. User information can be recorded in or reproduced from the data area 40. Other information related to the disc 1 can be recorded in the lead-out area 50. Reference numeral 25 denotes a stack ring which prevents productivity reduction due to contact of a recording plane when stacking the manufactured disc in a disc injection molding.
A disc chucking apparatus for reading/reproducing the disc 1 is shown in FIG. 2. The disc chucking apparatus includes a spindle motor 60, a turntable 63, and a clamper 65. The spindle motor 60 rotates the disc 1. The turntable 63, on which the disc 1 is securely placed, is fixed to a shaft (not shown) of the spindle motor 60. The damper 65 clamps the disc 1 on the turntable 63. The disc 1 is clamped between the turntable 63 and the damper 65, and thus information can be reproduced from the disc 1 without swaying when the disc 1 rotates.
A protrusion 63a, which protrudes from the center of the turntable 63, is inserted into the central hole 10. A magnetic substance 64 is included in the turntable 63 or the damper 65 so as to fix the disc 1 by a magnetic force of the magnetic substance. Here, the clamping area 20 of the disc 1 contacts the damper 65.
In a general CD or DVD having a whole external diameter φt of 120 or 80 mm, a diameter of the central hole 10 is standardized to be 15 mm. An external diameter φc of the clamping area 20 is standardized to be about 32.7 mm.
Typically, the stack ring 25 between the clamping area 20 and the lead-in area 30 has an area size in a range of 6–12 mm. A diameter of the lead-in area 30 of the CD is standardized to be 46–50 mm and a diameter of the lead-in area 30 of the DVD is standardized to be 45.2–48 mm. Therefore, data area 40 of the CD and DVD has the internal diameter φd of 48.2 mm. In a case of a DVD+RW, the internal diameter φi of the lead-in area 30 is determined to be 44.0 mm. In a case of a disc having a diameter of 120 mm, an external diameter φoe of the lead-out area 50 is determined to be up to 117 mm. In a case of a disc having a diameter of 80 mm, an external diameter φoe of the lead-out area 50 is determined to be up to 78 mm.
In a case where a disc has a small diameter according to a trend of compacting the disc, e.g., in a case where the disc has a whole diameter of 64 mm, data recording capacity is not enough if, like a CD or DVD, a position φd where user data starts being recorded is 48.2 mm of the whole diameter φt of the disc (i.e., typically at most a data area size of about 12.8 mm). Furthermore, a data area substantially hardly exists if the above-described standard is adopted to a disc having a whole diameter φt of 50 mm (i.e., at most a data area size of about 0.8 mm). If the central hole 10 of a disc is smaller than 15 mm to compensate for the insufficient data area, a disc having a diameter of 120 or 80 mm would not be compatible with a standard or typical disc drive.
There is another method of reducing the size of the clamping area 20 to secure a recording capacity of a disc. The size of the clamping area 20 depends on deflection components of the disc, the number of rotations of the spindle motor 60, or a clamping force of the disc. However, because vibration characteristics of the disc are deteriorated with a reduction in the clamping area 20, there are certain limitations in reducing the clamping area 20. The clamping area 20 has a diameter φc up to 33.0 mm according to a disc chucking standard. In particular, the spindle motor 60 for most of CD-ROMs or DVD-ROMs has an external diameter of 28.0 mm or less and the turntable 63 has an external diameter of 33.0 mm or less, which can satisfy a clamping area 20 diameter of up to 33.0 mm of the disc.
Therefore, in a case where a compact disc is provided taking into consideration portability (compatibility), recording capacity is reduced with a reduction in the diameter of the disc and sufficient data cannot be recorded on the disc. Also, by reducing the diameter of the disc, repeated recording is limited, and thus limiting free editing of the data, which is an obstacle to adding value to the data. Accordingly, a new standard of high-density optical discs is required so that high-density recording capacity is secured and compact discs are compatible with existing disc reproducing drives.